Many vehicles utilize catalysts in exhaust systems to reduce emissions. In lean exhaust conditions, such as with regard to diesel exhaust or other lean burning conditions, a catalyst may utilize reductant other than burnt fuel. One such aftertreatment device is a Selective Catalytic Reduction (SCR) system, which uses a catalyst to convert NOx to nitrogen and water. A urea-based SCR catalyst may use gaseous ammonia as the active NOx reducing agent, in which case an aqueous solution of urea may be carried on board of a vehicle, and an injection system may be used to supply it into the exhaust gas stream.
At ambient temperatures of less than −11° C., the aqueous urea solution (comprising 32.5% urea and 67.5% water) may freeze in the on board urea storage tank. Thus, a pick up tube of the injection system may not be able to deliver urea to the injector for delivery to the exhaust gas and NOx reduction. In one approach, the urea storage tank includes an electric heating system to warm the frozen urea. Further, components of the urea storage tank and urea injection system may have a freeze-safe design to assure functionality and survivability of the injection system over multiple freeze/thaw cycles.
The inventors of the present application have recognized a problem in such previous solutions. First, there may be increased cost associated with the heating and freeze-safe components for the urea storage tank and urea injection system. Second, fuel economy may be decreased by using energy produced by the vehicle to heat the entire urea tank, and such heating may take an extended duration, thus reducing the amount of exhaust gases that can be treated catalytically with the reductant, and thus increasing exhaust emissions overall.
Accordingly, in one example, some of the above issues may be addressed by an exhaust system for an engine, the exhaust system including a liquid reductant injection system, and a method for operating the liquid reductant injection system, wherein the method comprises storing a reductant mixture of ethanol, water, and urea; drawing the stored mixture into an electrically heated pick-up tube for delivery via a delivery line to the exhaust by operating a pump in a first direction; and, clearing a return line that returns the stored mixture or the delivery line by operating the pump in a second, reverse, direction.
In this way, by including ethanol in the reductant solution, a freezing point temperature of the liquid reductant may be reduced. As such, the occurrences of reductant freezing may be reduced. If the temperature drops below a precipitate forming threshold, the liquid reductant may be heated by the pick-up tube. Thus, the liquid reductant injection system consumes less energy because heating of the storage tank is not required. Because of these features, the system may require less energy during operation and overall fuel economy may be improved. Further still, pumping of the liquid reductant injection system in a reverse direction may clear delivery and return lines of remaining liquid reductant. This may prevent reductant from precipitating and/or freezing in the delivery and return lines during ambient temperature conditions below the liquid reductant precipitation and freezing points, thereby reducing clogging and improving efficiency of NOx reduction.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.